Coin discrimination sensors have been employed to discriminate among various coins. A typical coin discrimination sensor includes at least one primary coil for inducing eddy currents in the coin to be analyzed. The primary coil receives an alternating voltage which correspondingly produces an alternating current in the coil. The alternating current flowing in the primary coil produces an alternating magnetic field through and around the coil as is well known in the art.
Characteristics of the alternating magnetic field depend upon a variety of factors including the frequency and amplitude of the voltage applied to the primary coil. The primary coil, also known as the excitation coil, inductively couples with a coin brought into proximity with the coil, thereby producing eddy currents in the coin being analyzed. Because the magnetic field from the primary coil is alternating, the corresponding eddy currents are alternating as well. The induced eddy currents are influenced by the characteristics of the coin being analyzed.
The magnitude of the eddy currents produced is influenced by the frequency of the alternating magnetic fields applied. High frequencies tend to create magnetic fields that penetrate near the surface of the coin, giving a better indication of a coin's surface area. Low frequencies tend to penetrate further into the coin, giving a better indication of a coin's volume. Coin discrimination sensors which employ eddy currents to discriminate among different coins typically use an excitation signal that is oscillating at a single frequency. Thus, coin discrimination sensors having a high-frequency excitation signal distinguish better among coins of different diameter. Conversely, coin discrimination sensors having a low-frequency excitation signal distinguish better among coins of different thickness. What is needed, therefore, is a coin discrimination sensor that uses a composite excitation signal so as to distinguish among coins having different compositions, thicknesses, and diameters.